The present invention is related to signal amplification, and in particular to a loop filter for a PWM based amplifier with minimum aliasing error.
Various types of electronic amplifiers exist for amplifying an electrical signal using one or more output devices such as transistors. Electronic amplifiers are commonly grouped into several classifications based on how they fundamentally operate and how they conduct electricity over the different portions of the input cycle. Class A amplifiers have one or more output transistors that conduct in their linear range over the entire cycle or waveform of the input signal. The phase response of a class A amplifier is quite linear and produces a high quality output, but the class A amplifier is extremely inefficient. The output transistors in a class A amplifier may continue to conduct a DC current even if the input signal is off. Class B amplifiers have two complementary output transistors or sets of output transistors, each of which conducts in the linear range for half of the input cycle and is substantially off for the other half of the input cycle. A class B amplifier is much more efficient than a class A amplifier, but is susceptible to crossover distortion when switching from one output transistor to the other. Because it takes a small but significant amount of voltage for the output transistors to start conducting, portions of the input waveform that have a lower voltage than this turn-on voltage will not be reproduced faithfully by the class B amplifier. Class AB amplifiers combine some features of both class A and class B amplifiers in order to minimize or eliminate crossover distortion while sacrificing some efficiency. The output transistors in a class AB amplifier do not turn off during their inactive phase, but are biased so that they continue to conduct just enough to remain turned on during their inactive phase. While frequently used, all of these amplifier classes that operate in their linear ranges for all or some of the input cycle are relatively inefficient. For example, the theoretical limit on efficiency in these amplifier classes ranges from a maximum of about 78.5% or π/4 down to 25% or lower efficiencies.
As signal processing techniques have improved, class D amplifiers have seen increased use. The output transistors in class D amplifiers operate in switching mode, with the transistors either turned on or off in their most efficient states. The input signal is encoded or modulated in the switched signal produced by the output transistors. In one modulation technique, the input signal is pulse-width modulated by the class D amplifier. The output signal produced by the class D amplifier has a constant amplitude and frequency, but the width of each pulse is varied based on the strength of the input signal. The switching rate establishes a sampling frequency that is typically many times greater than the frequency of the input signal in order to capture the significant information. The output signal may be recovered or demodulated by passing it through a lowpass filter that effectively averages the PWM signal out. Considered at the switching frequency of the class D amplifier, the output of the lowpass filter appears to be a DC signal having an amplitude that is proportional to the duty cycle of the PWM output. Considered at lower frequencies, the output signal may appear as an analog signal having a widely varying amplitude, such as with the case of an audio signal. The class D amplifier is an attractive option because it is much more efficient than other amplifiers such as class A, class B or class AB amplifiers. The theoretical maximum efficiency of a class D amplifier is 100%, with actual efficiencies of over 90% depending on the application. Class D amplifiers are therefore used in a wide range of applications, such as audio amplifiers, motor control systems and other power conversion systems. This efficiency also results in lower cooling requirements, reducing the size and cost of cooling fins and housings for the amplifiers.
Many class D amplifier designs, however, suffer from distortion that limits their use in applications that require a high quality output such as high end audio amplifiers. A typical class D amplifier includes a feedback loop that introduces high frequency residuals into the signal at the PWM switching frequency. Distortion therefore remains a problem for class D amplifiers limiting their application in applications requiring a high quality output.
Hence, for at least the aforementioned reasons, there exists a need in the art for a loop filter for a PWM based amplifier that minimizes distortion from the high frequency modulation.